Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in geological formations in the Earth's crust. Wells are typically drilled using a drill bit attached to the lower end of a “drill string”.
Drilling fluid, or mud, is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and carries drill cuttings from the borehole back to the surface.
In various oil and gas exploration operations, it is beneficial to have information about the subsurface formations that are penetrated by a borehole created by the passage of the drill bit. These measurements may be essential to predicting the production capacity and production lifetime of the subsurface formation.
Samples may need to be taken of the formation rock within the borehole. A coring tool is used to take a coring sample of the formation rock within the borehole.
A typical coring tool usually includes a hollow coring bit which comprises an annular cylindrical cutting surface. The coring tool penetrates into the formation such that a coring sample enters in a hollow cylindrical section. When the hollow center of the coring tool is filled with the core sample, the coring tool is brought to the earth's surface to retrieve the core sample for analysis.
The sample is analysed to assess, amongst other things, the reservoir storage capacity (porosity) and the permeability of the material that makes up the formation surrounding the borehole, such as the chemical and mineral composition of the fluids and mineral deposits contained in the pores of the formation. The information obtained from analysis of a core sample may be used to make exploitation decisions.
Downhole coring operations are generally axial coring or sidewall coring.
In axial coring, the coring tool is disposed at the end of a drill string within a borehole, in which the coring tool may be used to collect a coring sample at the bottom of the borehole.
In sidewall coring, the coring bit from the coring tool may extend radially from the coring tool, in which the coring tool may be used to collect a coring sample from a side wall of the borehole.
An axial coring tool is an assembly of an inner barrel, an outer barrel and an annular core bit located at a core engaging end of the coring tool. Located opposite to the core engaging end is an attachment end of the coring tool.
At the attachment end of the coring tool, the inner barrel and the outer barrel are connected to a top sub. The outer barrel is connected to the outer diameter (OD) of the top sub through a stabiliser. The inner barrel is connected to the inner diameter (ID) of the top sub through a swivel assembly.
The swivel assembly includes a bearing which restricts the inner barrel from rotating when the outer barrel is rotated by the rotating the drill pipe/string. The top sub is connected to the end of the drill string through a threaded connection.
Drilling fluid or mud is pumped down the center of the drill pipes which form a drill string. Upon reaching the coring assembly, the drilling fluid passes through the inner barrel as well as the annulus between the inner barrel and the outer barrel.
The drilling fluid exits through the inner barrel and the ports in the core bit. The drilling fluid is passed through the inner barrel to clear the inner barrel. The drilling fluid is passed through the annulus between the inner barrel and the outer barrel and out of the ports of the coring bit in order to cool and lubricate the coring bit.
The drilling fluid is returned to the surface from the annulus between the coring tool/drill pipes and the bore hole. The returning drilling fluid carries with it formation cuttings from the drilled hole.
Prior to commencing coring, in typical applications a steel ball is dropped down the drill pipe such that it rests in the swivel assembly in order to block flow of fluids to the inner barrel and divert flow to maintain the flow of fluid in the annulus between the inner barrel and the outer barrel. The steel ball is captured in a lower portion center pipe of the swivel assembly (lower portion being below the bearing of the swivel assembly), just above the inner barrel.
The steel ball when in position at the swivel assembly creates a one-way valve to allow fluid/pressure build-up within the inner core barrel during coring to be relieved, but to prevent fluid passing down into the inner core barrel during such coring.
The coring assembly is positioned at the surface of the formation from where the formation sample is to be obtained. The core bit is rotated by rotating the outer barrel which may be rotated by rotating drill pipe. The inner barrel is kept stationary. The rotation of the core bit and the weight on bit causes the coring tool to penetrate the formation. A core sample, positioned between in the annulus of the core bit, enters the inner barrel as the coring tool advances into the formation. Once the inner barrel is filled with core samples rotation of the core bit is ceased.
A core catcher, in some instances spring loaded, grips the core sample from below the inner barrel. As the coring tool is lifted, the core sample breaks just below the core catcher. The coring assembly is then pulled out of the hole to the surface to retrieve the core sample.
For unconsolidated formations, such as heavy oil sands, which present a risk of sliding out of the inner barrel during the travel to surface, a full closure type system (FCS) is deployed. An FCS system has a mechanism which seals the bottom of the inner barrel, for example by a collapsible shoe or mechanically activating the closure or sealing the bottom of the core, so that the captured core does not slip out of inner barrel. In a mechanical or collapsible shoe mechanism, once the inner tube is filled with core sample, the shoe collapses or seals blocking the bottom portion of the inner tube to prevent the core sample from sliding out of the inner tube. Such sealing mechanisms may replace the core catcher.
If an FCS or alternative system is used, a further steel ball which is dropped down the drill string has a second important function. Apart from acting as a one-way valve blocking flow of mud down the inner barrel, this second steel ball activates the FCS mechanism or alternative system to activate and seal the lower portion of the inner barrel preventing core from falling out of the core barrel.
Such standard coring methods provide no feedback to the operator. The operator has only an ambiguous indication of whether the core column is entering the inner barrel, inside the inner barrel, or has fallen out of the barrel. If the operator's judgement is incorrect, the coring operation can become extremely expensive and time consuming.
For example, if the operator considers that the core sample is in the inner barrel, when in fact the core sample has fallen out, the reality is only confirmed after the coring tool is retrieved to the surface.
Another example of incorrect functioning of the coring tool is ‘core jamming’. The core formation can jam inside the inner barrel such that further core does not enter the inner barrel while the coring tool is working on the formation. If undetected, the core bit will merely mill the formation without obtaining full core.
Also, the coring equipment may get damaged because of core jamming. The time taken for retrieving the coring tool and a second round of coring is a few days on the rig.
As an estimate, the additional time spent due to the delay, in present day terms, amounts to millions of dollars of costs.
One or two drill operators have used expensive sensors in conjunction with a Mud Pulse Telemetry (MPT) system to provide feedback to the operator. Such sensors detect core capture and/or core fall out and provide a signal to a mud pulser which transmits the signal to the surface.
One such sensor is described in WO 2011020141 A1 published on 24 Feb. 2011. The contents of WO 2011020141 A1 are incorporated in their entirety in this patent application by reference.
The MPT system is a common method of data transmission used for Measuring While Drilling (MWD) tools. Down hole, a valve or a mud pulser” is operated to restrict the flow of the drilling mud according to the digital information to be transmitted. This creates pressure fluctuations representing the information. The pressure fluctuations propagate within the drilling fluid towards the surface where they are received from pressure sensors. On the surface, the received pressure signals are processed by computers to reconstruct the information.
The three types of MPT systems are positive pulse, negative pulse and continuous wave.
Positive MPT uses a hydraulic poppet valve to momentarily restrict the flow of mud through an orifice in the drill pipe to generate an increase in pressure in the form of positive pulse or pressure wave which travels back to the surface to be detected.
Negative MPT uses a controlled valve to vent mud momentarily from the interior of the drill pipe into the annulus between the drill pipe and the bore hole. This process generated a decrease in pressure in the form of a negative pulse or pressure wave which travels back to the surface to be detected.
Continuous wave telemetry uses a rotary valve or “mud siren” with a slotted rotor and stator which restricts the mud flow in such a way as to generate a modulating positive pressure wave which travels back to the surface to be detected.
There are other types of telemetry systems such as electro-magnetic (EM) system and induction system. An EM system applies voltage into the earth's crust, using it as a conductor. An EM system is cheaper than mud pulse system. However, an EM system is not suitable for use offshore where the EM signal does not pass through water. An induction system is suitable for use offshore. However, an induction system uses proprietary drill pipes having end connections to transmit signals from one drill pipe to another, and wired connection between two end connections of a drill pipe. These specialised drill pipes are expensive and in most operations they are cost prohibitive.
A standard MPT system is primarily designed for a drilling operation and not for coring operation. During drilling, the mud pulser is installed proximate to the drill bit.
Likewise, one or two operators (mentioned earlier) who have used sensors in conjunction with MPT have installed such mud pulser adjacent to the coring tool assembly. To do so, the sensors were placed in the coring assembly. An adjustable electrical coupling, connected to the sensors, protrudes out of the swivel assembly of the coring tool.
A plurality of flow subs that are designed specially for the mud pulser to operate are held above the coring tool having the sensors. These flow subs are different to regular drill pipes which form the drill string. The flow subs are made to suit the function of the mud pulser.
The electrical connection is made between the sensors and the mud pulser. The flow subs are lowered and screwed into the core assembly. Once assembled, the mud pulser is turned on via a download port provided on the periphery of one of the flow subs. Subsequently, the drill pipes are attached to the end of the mud pulser flow subs to form a drill string.
Once drilling fluid is pumped down the drill string, the pulser relays data from the sensors to the top of the drill string. The drilling fluid passes through the mud pulser to the coring tool.
There are many difficulties with this methodology.
Firstly, it is difficult to physically connect the adjustable electrical coupling of the sensor protruding from the coring assembly to the expandable electric coupling of the mud pulser.
It is very difficult to make the connection physically particularly on an off-shore rig because the platform of the off-shore rig is not steady. The person making the electrical connection has to place his hands between the core assembly and the heavy flow subs of the mud pulser suspended above the core assembly. This installation method increases the risk of accidents on the rig.
Secondly, the flow subs used with mud pulser are heavy and expensive because of their thickness and proprietary design. The proprietary flow subs are designed to be used with a mud pulser. They form a part of the Bottom Hole Assembly (BHA) and so they need to be thick in order to provide sufficient weight on the coring bit. This adds to the capital costs of the rig.
Thirdly, the flow subs of the mud pulser require a lot of critical maintenance. Particularly, their end threads need to be inspected after every job by a service company who provides the mud pulser. Such external inspections are expensive.
Further, the additional connections of flow subs required using existing method can increase the chance of tool failures. This adds to the cost of coring operation.
Also, time spent on-site on installing the MPT system and maintaining it adds to the cost of operating the drill rig.
Finally, an FCS system is not useable with such a system because it is not possible to drop a ball to the swivel assembly of a coring tool as the mud pulser blocks the passage of the ball.
So it is not possible to use the currently available FCS type systems in the aforementioned method.